It is well known in the hydrocarbon production industry to line wells with a number of concentric tubes to stabilise the bores and reduce the risk of fracture of the formation while drilling with weighted mud. The top section of the well casing is generally known as the conductor pipe or conductor or sometimes as the structural casing. This conductor is essentially the well foundation and has different functions throughout its life. The main requirements of the conductor are to:                (1) Stabilize and protect the near-surface sediments during the initial top-hole drilling operations by preventing well bore collapse and fracture of the formation by mud pressures.        (2) Temporarily support the weight of the next section of casing string, usually known as the surface casing.        (3) Act together with the cemented surface casing to resist temporary axial tensile loads, shear loads and bending moments transmitted from the riser through the Blowout Preventer (BOP) while drilling the remaining sections of well.        (4) Act together with the surface casing to resist longer term operational loads from well trees, production risers and other production facilities.        
Onshore, or in shallow water, the conductor may be driven into the ground using piling techniques. However, it can be difficult to pile drive a conductor through a relatively thick or deep layer of sand. Installing conductor in deep water presents additional difficulties. One method that has been proposed uses “toe-driving” of the conductor. In this method, the casing is driven into the soil by the impact of a hammer acting at the bottom or toe of the pile compared to driving with a hammer striking at the top of the pile. In one example, a specially designed tip comprises an anvil and a conical penetrating tip. The hydraulic hammer, arranged inside the casing, hits the anvil and drives the conical tip into the soil. The casing is attached to the tip not rigidly but by a shock absorbing element which acts to pull the casing into the soil after the tip. The toe driving method has not been widely used for deep water applications as it can be difficult to achieve deep penetration.
The installation of conductors in deep water usually comprises forming an oversize hole by drilling and then cementing the conductor in place or by jetting, a procedure similar to wash boring in onshore geotechnical investigations. Although the drill and cement process is widely used in relatively shallow water, the soil in deepwater can be relatively weak. Consequently, there is a tendency for the drilled hole to be filled up by soil collapsing from the sides, not only while drilling is being carried out, but also when the drillstring is pulled out for running in casing. Therefore, jetting has been the more commonly used method for deep water. Jetting is often preferred as it may be quicker and, when performed properly, more reliable than drilling and cementing in normally consolidated clay sediments. However, if not properly planned and executed, jetting may result in excessive delays and even abandonment of a well. One of the greatest risks is excessive disturbance and wetting of the soil that results in low axial capacity and excessive settlement of the conductor when the surface casing is landed on it.
In a conventional jetting method a drill pipe is run inside a length of conductor and locked to the conductor by a drill-ahead running tool. The section of drill pipe inside the conductor, generally called the bottom hole assembly may typically comprise a drill bit positioned close to the lower end of the conductor, a mud motor, measurement-while-drilling directional sensors and a series of heavy drill collars with spacers. The assembly is lowered to the seabed and the conductor is washed into place with seawater and high viscosity mud sweeps injected through the drill bit. The pressurised fluid erodes the soil inside the conductor and around the lower edge and flushes the cuttings up the annular space between the conductor and bottom hole assembly. The resulting reduced soil resistance allows the jet assembly to penetrate the seabed under the combined weight of the conductor and bottom hole assembly. If this weight is insufficient to overcome the frictional resistance, the conductor may be worked up and down (reciprocated) by several metres to wet and remould the surface of the borehole.
As the outside surface of the conductor is in direct contact with the surrounding soil throughout the installation, there may be no need for a cementing operation with jetting. The installation of the conductor relies on the surrounding soil collapsing around the outside of the conductor to support the weight of the conductor and subsequent casing strings. Often jetting is only possible to a relatively shallow depth, typically around 80 metres below mud line, as beyond this, the soil may not collapse around the conductor sufficiently to provide the required support or may take an unacceptably long time to do so. Another potential problem of the known jetting processes is that if the soil does not sufficiently consolidate around the conductor, it may not provide adequate formation isolation. This can be important when shallow formation is warmed up during subsequent drilling or production and hydrate gasifies and rises through the weak soil around the conductor to the seabed, possibly further weakening the soil and reducing its load bearing capacity.